GNSS Broadcast Ephemeris Errors Analysis And Real-time Correction Method Based On Precise Ephemeris

The GNSS satellite ephemeris is the precondition and foundation of satellite navigation and positioning.At present,GNSS satellite ephemeris can be mainly divided into broadcast ephemeris and precise ephemeris.GNSS broadcast ephemeris is widely used in real-time navigation and positioning due to its features of real-time and easy-to-access.Precise ephemeris has high precision but hysteresis,which is mainly applied into high-precision scientific research.Compared with the GNSS precise ephemeris,the shortcomings of low accuracy in broadcast ephemeris limit the further improvement of real-time navigation and positioning accuracy.Therefore,this paper analyzed the orbit and clock errors of the GNSS broadcast ephemeris,and studied the real-time correction method of its orbit.The main research contents and conclusions are as follows:1)The accuracy of GNSS broadcast ephemeris was analyzed.Based on the five-year data of GPS,GLONASS,Galileo,BDS,QZSS broadcast ephemeris and corresponding precise ephemeris from 2014 to 2018,the changes of broadcast ephemeris accuracy of each system was analyzed by calculating its daily root mean square of orbit and clock errors in each system.The results show that the accuracy of the orbit and clock of GPS broadcast ephemeris are the best and the most stable in five years.GLONASS orbit accuracy is better,but its clock accuracy is poor.Thanks to plenty of launches and operations of the Galileo FOC satellite,the accuracy of the broadcast ephemeris orbit and clock errors have improved significantly,and there is a tendency to catch up with even surpass GPS.BDS broadcast ephemeris orbit and clock accuracy are worse,and the dispersion is also greater.QZSS broadcast ephemeris orbit and clock accuracy are better than BDS's,but its fluctuation is larger.The accuracy of broadcast ephemeris from different systems was compared by calculating the overall accuracy index SISRE of the broadcast ephemeris.The results show that the order of the accuracy of these five system broadcast ephemeris is Galileo>GPS>QZSS>BDS>GLONASS.The accuracy of a single satellite broadcast ephemeris was analyzed by calculating root mean square of orbit errors,clock errors,the SISRE(orb)and SISRE.The results show that the radial errors and clock errors are the main contributing factors of SISRE(orb)and SISRE,respectively.2)GNSS broadcast ephemeris orbit errors noise characteristics and statistical Characteristics were analyzed.The noise characteristics of GNSS broadcast ephemeris orbit errors were analyzed by the maximum likelihood estimation method.The results show that the noise types of orbit errors from different systems are different,among which colored noise dominates.The statistical characteristics(mean,standard deviation,correlation,distribution model)of the broadcast ephemeris orbit errors were analyzed.The results show that only the mean of the normal errors is zero meters.All satellites have significant nonzero mean for their radial errors,tangential errors and clock errors,but their mean is still within±1 time standard deviation.On the whole,the correlation only exists among radial errors,tangential errors and clock error for GPS and QZSS satellites.There is a weak correlation between the radial errors and tangential errors of GLONASS satellites.Galileo and BDS satellite radial errors have correlation with tangential errors and clock errors,respectively.In particular,the GPS IIF,BDS MEO satellites have no correlation among radial errors,tangential errors,while the QZSS GEO satellite has a significant correlation with them.Only a few satellite orbit and clock errors obey the sub-Gaussian distribution,and all others submit to super Gaussian distribution.3)The real-time correction model of GNSS broadcast ephemeris was studied.It is proposed to construct an orbit errors sequence by using Ultra-Rapid precise ephemeris(observed)and corresponding broadcast ephemeris to establish a real-time correction model of GNSS broadcast ephemeris based on SSA.Taking GPS satellite as an example,the feasibility of this model is analyzed by experiments.The results show that the orbit accuracy of broadcast ephemeris has been improved,which proves that the model is feasible.